It is often desired to distribute a multiplicity of radio signals with similar modulation formats to a bank of demodulators for processing. For example, the hub station (HS) of a star configured satellite network may receive signals on a number of different frequency channels. Each of these channels may have a multiplicity of signals that employ frequency division multiple access (FDMA), time division multiple access (TDMA) or code division multiple access (CDMA). Further, the signals may be received on multiple antennas to provide spatial diversity. In addition, the antenna system may be duplicated so that more than one satellite can be received simultaneously.
The straight forward approach of having a demodulator configured for each possible link that may be received is often impractical due to the large number of links. Further, reliability considerations would generally dictate that each demodulator be duplicated, which is very inefficient, in order to minimize the number of demodulators required, it is desirable to be able to assign available demodulators to active channels. This is especially important when the total number of users supported is considerably less then the number of link assignments available. This may be the case, for example, if a satellite with a mufti-beam antenna is used in a nonsynchronous orbit as proposed for several of the satellite based personal communications-systems (PCS). In this instance, the beams may illuminate areas with very different user densities. Thus, at times it is necessary to assign a large number of demodulators to some beams and only a few to others. Unfortunately, the mix changes with time as the satellite moves, so dynamic allocation is required.
The problem of switching a large number of demodulators over a larger number of link assignments is not trivial, and a tradeoff must occur between the hardware saved by dynamic assignment and the hardware required to perform this operation. This tradeoff usually results in demodulators being "pooled" to be switched in groups, often with a redundant pool (of lesser size). The efficiency and flexibility of demodulator usage is primarily related to the size of the minimum number of demodulators that can be assigned to a particular channel. In general, greater flexibility and efficiency requires more complex switching: This switching generally employs analog techniques which limits the flexibility that call be obtained.